National inpatient complications and outcomes after surgery for spinal metastasis from 1993–2002

Abstract

BACKGROUND.

Information regarding patient outcomes, complications, and mortality after surgery for spinal metastasis has previously been derived from single-institution series. The aim of this study was to report inpatient mortality, complications, and outcomes on a national level.

METHODS.

The National Inpatient Sample (NIS) was utilized to identify 26,233 admissions of surgically managed spinal metastasis in the U.S. from 1993 through 2002. Multivariate analysis was performed to analyze the effects of patient and hospital characteristics on outcomes such as mortality, complications, discharge disposition, and length of stay.

RESULTS.

The in-hospital mortality rate was 5.6% and the complication rate was 21.9%. Pulmonary (6.7%) and postoperative hemorrhages or hematomas (5.9%) were the most common complications reported. A single postoperative complication increased the mean length of stay (LOS) by 7 days and the mortality rate by 11%. Multivariate analysis showed that complications were more likely in older patients and in patients with 2 or more comorbidities. With patients having no comorbidities as the reference group, 1 comorbidity increased the risk of in-hospital death by almost 4-fold. Mortality was significantly higher in men, in patients who had postoperative complications, and in patients who were operated in the earlier (1993–1997) year group.

CONCLUSIONS.

A national perspective is provided on inpatient complications and outcomes after surgery for spinal metastasis in the U.S. The significant negative effect of postoperative complications on mortality and resource utilization is demonstrated. Furthermore, preoperative comorbidity is identified as an important risk factor and its impact is defined on patient outcomes. Cancer 2007. © 2007 American Cancer Society.

Spinal metastases represent an important challenge for spine surgeons. Systemic neoplasia from a multitude of cancer types can spread to the spine and cause significant pain and debilitation. As the treatment of many primary tumors improves and survival is enhanced, more patients are ultimately experiencing metastases to the spinal column.1 In fact, the vertebral column is the most common site of bony metastasis, accounting for 18,000 new cases in North America annually.2 Tumor types most likely to metastasize to spine include breast, lung, prostate, kidney, and hematopoietic cancer.1 Once metastasis to the spinal column has occurred, treatment is palliative. It is aimed at reducing the tumor burden, alleviating pain, and preserving or restoring stability and neurologic function.

The efficacy of surgical procedures is assessed via evaluation of postoperative outcomes. This analysis is critical to the decision-making regarding treatment recommendations and the practice of evidence-based medicine. Thus, it is extremely important to carefully examine surgical outcomes on an institutional as well as national level.

Most information regarding patient characteristics, morbidity, and mortality after surgery for spinal metastasis has been derived from single-institution series.3–6 The aim of the current study is to define the inpatient mortality rate, complication rate, and the nature of complications after surgery for spinal metastasis on a national level. We also analyze the effects of patient characteristics and hospital volume on outcomes such as complication rate, mortality rate, discharge status, and length of stay.

MATERIALS AND METHODS

Data Source

Data were obtained from the National Inpatient Sample (NIS), which is the largest all-payer inpatient care database.7 It is maintained by the Agency for Healthcare Research and Quality (AHRQ) and is used by policy makers to identify, track, and analyze national trends in healthcare use, access, charges, quality, and outcomes. It contains data from approximately 8 million annual discharges from 1004 hospitals in 37 states. It represents a 20% stratified sample of nonfederal community hospitals, representing approximately 85% of all hospital discharges in the U.S.

Inclusion Criteria

All patients from 1993–2002 who had a primary diagnosis of secondary malignant neoplasm of the spinal cord/brain, meninges, or bone (International Classification of Diseases [ICD-9] diagnosis codes: 198.3, 198.4, 198.5) who also underwent spine surgery (ICD-9 Primary Procedure codes: 03.0, 03.4, 03.09, 81.0, 81.00–81.08) were included in this study.

Patient and Hospital Characteristics

Independent variables such as patient age, sex, race, comorbidities, hospital size, and year of treatment were abstracted from the NIS. Age was categorized into the following 5 groups: age birth to 17 years, 18 to 44 years, 45 to 64 years, 65 to 84 years, and >84 years. Race was recategorized into white, black, and other. Year of treatment was categorized into 2 groups: 1993 through 1997 and 1998 through 2002. The frequency of a set of medical comorbidities as described by Elixhauser et al.8 and listed in Table 1 were calculated using the AHRQ software (available at URL: www.hcup-us.ahrq.gov/tools_software.jsp [Accessed January 5, 2007]).9 A single comorbidity score was then derived and used in the analysis.

Table 1. List of Comorbidities and the Percentage of Patients With Surgically Managed Spinal Metastasis Who Were Reported to Have These Comorbidities

Comorbidity	Percentage of patients with the comorbidity
HIV indicates human immunodeficiency virus; AIDS, acquired immunodeficiency syndrome.
Hypertension	25.1
Chronic lung disease	12.8
Diabetes mellitus without complications	7.9
Coagulation deficiency	3.8
Congestive heart failure	3.2
Depression	2.7
Weight loss	2.1
Heart valve disease	1.8
Obesity	1.6
Peripheral vascular disease	1.4
Alcohol abuse	1.2
Diabetes mellitus with chronic complications	1.1
Renal disease	1.1
Psychoses	1.1
Liver disease	0.8
Lymph	0.8
Rheumatoid arthritis	0.6
Drug abuse	0.5
Pulmonary circulation disease	0.1
HIV/AIDS	0.1
Chronic peptic ulcer disease	0.03

Annual hospital volume was determined using the unique hospital identifier for each institution. Annual hospital volume was then divided into quintiles representing very low, low, medium, high, and very high volumes. The numeric thresholds for volume were 1 to 4 cases per year (very low), 5 to 9 cases per year (low), 10 to 16 cases per year (medium), 17 to 31 cases per year (high), and >31 cases per year (very high).

Outcome Variables

The primary outcomes were mortality and complications. Complications were obtained using the following ICD‒9 codes: neurologic complications (997.00–997.09); pulmonary complications (518.81–518.85, 997.3); thromboembolic complications including deep venous thrombosis (DVT) and pulmonary embolism (415, 387, 415.11–415.19, 4510–4519, and 4530–4539); cardiac complications (997.1 and 410); urinary and renal complications (584 and 997.5); hemorrhage or hematoma complicating a procedure (998.1–998.13).

Adverse outcome was defined as death or discharge to institution other than home. Adverse outcome, length of stay (LOS), and hospital charges were also abstracted from the NIS dataset.

Statistical Analysis

Descriptive statistics for demographic variables were conducted. Outcomes were compared with chi-square analysis for categorical variables. A P value < .05 was accepted as significant. A multivariate logistic regression model was constructed for the multivariate analysis, and only those variables that satisfied the screening cutoff of P < .1 with the univariate analysis were incorporated into the multivariate analysis. The multivariate odds ratios (ORs) are reported with the 95% confidence intervals (CIs). For the purpose of multivariate analysis, LOS was categorized into normal LOS (less than or equal to median LOS) and prolonged LOS (greater than median LOS) and admissions with missing race information were imputed as white. Multivariate analysis was performed for the following binary outcome variables: mortality, complications, adverse outcome, and LOS. Extrapolations to the entire U.S. population were performed using the SAS PROC SURVEY methodology.7 All data analyses were performed using SAS software (version 9.1; SAS Institute, Inc, Cary, NC) running on windows XP_Pro.

RESULTS

Using the NIS, a total of 26,233 admissions for surgically managed spinal metastasis were identified from 1993–2002 in the U.S. Patient characteristics and their association with morbidity are presented in Table 2. Forty-four percent of the patients treated were ages 45 to 64 years. Eighty-six percent of patients were white and 59% were women. Only 13.5% of patients had no comorbidities and 20% had ≥3 comorbidities. The majority of patients (75%) were treated at large hospitals. Total in-hospital complication rate was 21.9% and the in-hospital mortality rate for the entire cohort was 5.6% (Table 3). The mean LOS was 12 days, the median LOS was 9 days, and the percent of patients discharged home was 54%.

Table 2. Patient and Hospital Characteristics and Their Associated Morbidity After Spinal Surgery for Spinal Metastasis in the U.S. From 1993–2002

	No. of patients	Percent of patients	Percentage of patients with a complication	P
* Statistically significant value. † Missing race was imputed as white.
Age, y				.02*
0–17	263	1.0	12.5
18–44	2774	10.6	13.1
45–64	11559	44.1	17.4
65–84	11256	42.9	18.8
>84	358	1.4	23.9
Sex				.6
Male	15512	59.1	17.8
Female	10721	40.9	17.2
Race				.12
White†	22620	86.2	17.2
Black	1841	7.0	21.6
Other	1771	6.8	17.4
Comorbidities				<.001*
0	3552	13.5	13.6
1	10768	41.0	15.5
2	6573	25.1	19.2
≥3	5339	20.4	22.2
Hospital volume				.017*
Very low	5228	19.9	16.8
Low	5886	22.4	16.4
Medium	5028	19.1	16.1
High	4844	18.5	16.6
Very high	5248	20.0	21.7
Hospital size				.43
Small	1452	5.5	14.6
Medium	5048	19.3	17.9
Large	19715	75.2	17.6
Time period of procedure				.35
1993–1997	12989	49.5	16.9
1998–2002	13244	50.5	18.1

Table 3. Summary of the Number of Cases and Patient Outcomes After Spinal Surgery for Spinal Metastasis in the U.S. From 1993–2002

Total no. of cases (discharges)	26,233
In-hospital death	5.6%
Total complication rate	21.9%
Percentage of patients with a complication	17.5%
Mean length of stay	12.0 d
Discharged home	54%
Discharged to another institution	46%
Mean total charges	$45,708

Table 4 classifies the complications by pulmonary, postoperative hemorrhage/hematoma, thromboembolic, urinary/renal, cardiac, infectious, and neurologic. Pulmonary complications (6.7%) and postoperative hemorrhage or hematoma (5.9%) were the most common complications reported. The effect of complications on mortality, LOS, and hospital charges is shown in Table 5. With just 1 postoperative complication the mean LOS increased from 10 days to 17 days, the mortality rate increased from 3% to 14%, and hospital charges increased by > $20,000.

Table 4. Categorization of Complications After Surgery for Spinal Metastasis

	Frequency	Complication rate (%)
DVT indicates deep vein thrombosis; PE, pulmonary embolism.
Pulmonary	1764	6.7
Postoperative hemorrhage/hematoma	1546	5.9
Thromboembolic (DVT + PE)	774	3.0
Urinary or renal	590	2.2
Cardiac	503	1.9
Infectious	414	1.6
Neurologic	158	0.6
All complications	5748	21.9

Table 5. Effect of Complications on Mortality, Length of Stay, and Hospital Charges

No. of complications	Mean length of stay	Mortality, %	Mean rotal charges, U.S.$
0	10.5	3.4	39,815
1	17.2	14.0	63,331
2	23.7	20.9	100,633
3	28.6	28.0	123,368

Univariate analysis identified age, comorbidities, and hospital volume as potential predictors of morbidity (Table 2). Multivariate analysis for complications demonstrated that morbidity was significantly higher for patients with older age and with ≥2 comorbidities (Table 6). Interestingly, the analysis also showed a 1.5 times higher complication rate in the ‘very high’ volume centers compared with the lower volume centers. Percent of patients at high volume centers with no comorbidities (12.2%) was similar to the percent of patients at low volume centers with no comorbidities (11.8%).

Table 6. Multivariate Analysis Demonstrating the Effect of Patient Characteristics and Hospital Volume on Complications After Surgically Managed Spinal Metastasis

Variable	Odds of a complication (OR)	95% CI
OR indicates odds ratio; 95% CI, 95% confidence interval. * Reference group for comparison. † Statistically significant value.
Age, y
0–17	1.1	0.4–2.6
18–44*	–
45–64	1.4†	1.1–1.8
65–84	1.5†	1.1–2.0
>84	2.1†	1.1–4.0
Comorbidities
0*	–
1	1.2	0.9–1.5
2	1.5†	1.2–2.0
≥3	1.8†	1.3–2.4
Hospital volume
Very low	0.6†	0.5–0.8
Low	0.6†	0.5–0.8
Medium	0.6†	0.5–0.9
High	0.7†	0.5–0.9
Very high*	–

Multivariate analysis showed a significant effect of age, sex, comorbidities, complications, and time period of procedure on mortality (Table 7). The effect of hospital volume on mortality was not significant in the univariate analysis and was thus excluded from the multivariate analysis provided in Table 7. Significantly lower in-hospital mortality was found in the patients ages birth to 17 years (OR = 0.001) and in females (OR = 0.7). In addition, a strong effect of comorbidities and complications on mortality was identified. With patients having no comorbidities as the reference group, 1 comorbidity increased the risk of in-hospital death by 3.7-fold and ≥3 comorbidities increased the risk of death by 4-fold. Similarly, patients with 1 complication were 4.6 times more likely to die compared with the patients with no complications. Patients operated in the earlier time period (from 1993–1997) were 1.6 times more likely to die in the hospital compared with patients operated in the later time period (from 1998–2002).

Table 7. Multivariate Analysis of Mortality After Surgically Managed Spinal Metastasis

Variable	Odds of death (OR)	95% CI
OR indicates odds ratio; 95% CI, 95% confidence interval. * Statistically significant value. † Reference group for comparison.
Age, y
0–17	0.001*	0.001–0.002
18–44†	–
45–64	0.7	0.4–1.1
65–84	1.1	0.7–1.8
>84	0.3	0.1–1.7
Gender
Male†	–
Female	0.70*	0.5–0.9
Comorbidities
0†	–
1	3.7*	1.9–7.0
2	3.8*	2.0–7.3
≥3	4.1*	2.1–8.0
Complications
0†	–
1	4.6*	3.4–6.1
2	7.8*	5.0–12.1
≥3	13.1*	6.7–25.5
Year group
1993–1997	1.6*	1.2–2.1
1998–2002†	–

Multivariate analysis revealed age, comorbidities, and complications as significant factors affecting adverse outcome (Table 8). Compared with the reference group (ages 18–45 years), patients ages 65 to 84 years were more than twice (OR = 2.5) as likely to have an adverse outcome (death or discharge to an institution other than home). Patients age >84 years were 7 times more likely to have an adverse outcome. The likelihood of an adverse outcome increased nearly 2-fold (OR = 1.8) with ≥3 comorbidities. Similarly, compared with patients with no complications, patients with 2 or more postoperative complications were 2.6 times more likely to have an adverse outcome. Multivariate analysis for length of stay showed increased LOS for patients who were operated on in the earlier year group of 1993 through 1997 (OR = 1.5), and who had ≥1 comorbidities or postoperative complications.

Table 8. Multivariate Analysis of Adverse Outcome After Surgically Managed Spinal Metastasis

Variable	Odds of an adverse outcome (OR)	95% CI
OR indicates odds ratio; 95% CI, 95% confidence interval. * Statistically significant value. † Reference group for comparison.
Age, y
0–17	0.3*	0.1–0.9
18–44†	–
45–64	1.2*	1.02–1.5
65–84	2.5*	2.0–3.0
>84	7.6*	3.8–15.4
Comorbidities
0†	–
1	1.4*	1.1–1.6
2	1.6*	1.3–1.9
≥3	1.8*	1.4–2.2
Complications
0	–
1	1.7*	1.4–2.1
2	2.6*	1.8–3.7
≥3	2.3*	1.3–4.0

DISCUSSION

Our study reports outcomes from a nationally representative sample of hospitals that performed spine surgery for spinal metastasis between 1993 and 2002 in the U.S. The present analysis included 26,233 such admissions with an in-hospital mortality rate of 5.6% (Table 3). Weigel et al.3 reported a 7% in-hospital death rate in their case series of 76 patients. Other single-institution case series have reported a 30-day mortality rate of 3% to 13%.2, 5, 10 The only previous study utilizing an administrative database was a regional study from Ontario, Canada that reported a 30-day mortality rate of 9%.11 This in-hospital mortality rate is quite high compared with other surgeries. For example, craniotomy for resection of metastatic tumors has an in-hospital death rate of 3.1%.9 This fact emphasizes not only the gravity of these procedures but also the fragility of the general health of patients undergoing spine surgery for spinal metastasis.

Previous single-institution studies have reported surgical complication rates of 18% to 48%.2, 3, 6, 12–15 The current analysis abstracted an inpatient complication rate of 21.9%. This is likely an underestimate of the true complication rate because only select complications were examined. In addition, the NIS database contains only inpatient complications for that particular admission. Furthermore, it is possible that some especially minor complications were underreported. We categorized the nature of postoperative complications after spine surgery for spinal metastasis. This analysis revealed that pulmonary complications and postoperative hemorrhage/hematoma were most common, followed by thromboembolic complications. Neurologic complications were reported in only 0.6% of the total cases.

To our knowledge, this is the first study to report morbidity, mortality, and the effect of postoperative complications on resource utilization on a national level for surgically treated spinal metastasis. Although it is intuitive that postoperative complications would lead to a worse outcome and increased resource utilization, this has not been quantified. This study shows that just 1 postoperative complication leads to a 7-day increase in the mean length of stay and quadruples the mortality rate (Table 5). Hospital charges also increase dramatically with the number of postoperative complications.

Multivariate analysis found that complications increased with age and multiple comorbidities. Interestingly, an inverse volume-outcome effect was observed, with the highest volume hospitals reporting the highest complications (21.7%) (Table 2). To explain this effect we compared the number of comorbidities listed for patients treated at the highest volume centers versus low volume centers. This analysis showed that the highest volume hospitals did not report a higher number of comorbidities compared with low volume hospitals. Curry et al.16 previously reported this same inverse volume-outcome effect for complications after craniotomy for meningioma in the U.S. and concluded that there were many possible explanations for this observation. First, the highest volume centers may have treated more aggressive or recurrent tumors. Second, high-caseload hospitals may have performed more aggressive resections or resections after radiation therapy. The limitations of the NIS data source do not allow us to distinguish between these and other possible explanations. Finally, because our univariate analysis did not show a volume effect on mortality or adverse outcome, this increased morbidity did not translate into worse mortality or discharge disposition for the very high volume hospitals. This ‘rescue phenomenon’ has been previously described where the high volume tertiary centers are better equipped to identify and deal with complications and ‘rescue’ patients from bad outcomes.17, 18

The majority of patients undergoing surgery for spinal metastasis had at least 1 comorbidity. Hypertension, chronic lung disease, and diabetes mellitus were the most common comorbidities listed. Multivariate logistic regressions demonstrated a strong negative effect of comorbidities on mortality, morbidity, discharge status, and length of stay. For example, patients with a single comorbidity were 3 times more likely to die and nearly 2 times more likely not to be discharged home. Prognostic models of survival after surgery for spinal metastasis have been developed.19 This study emphasizes and supports the use of comorbidity data in all such prognostic models that guide preoperative patient selection.

After controlling for factors such as age, sex, and comorbidities in the multivariate analysis, postoperative complications were found to increase mortality significantly. For example, patients with 1 complication had a 4.5-fold chance of dying compared with patients with no complications. Decreasing the complication rate is thus extremely important for the successful surgical management of patients with spinal metastasis.

In summary, we believe the current study provides a national perspective on inpatient complications and outcomes after surgery for spinal metastasis in the U.S. The strong negative effect of postoperative complications on mortality and resource utilization is demonstrated. In addition, preoperative comorbidities and postoperative mortality rates were shown to be quite high for these procedures. Furthermore, we identified preoperative comorbidity as an important risk factor and defined its impact on patient outcomes.